Housing in a Large Open Cage Did Not Affect the Phenotypic Traits of Obese Male Zucker fa/fa Rats When Compared to IVC-Housed Rats, but Improved the Rats’ Well-Being

The housing of laboratory rats in cages with dimensions according to international standards for research animals can hardly be regarded as a stimulating environment, even when fulfilling the minimum requirements for environmental enrichment. Little is known about whether changes in the housing situations to improve living conditions will affect the phenotypic traits of well-known models for human diseases. The obese Zucker fa/fa rat develops hyperlipidemia, hypertension, and fatty liver, and is widely used for studies on metabolic complications of obesity in humans. Young male obese Zucker fa/fa rats were housed in pairs in standard individually ventilated cages (IVCs: floor area 1500 cm² and maximum height 20 cm), or 4-6 rats were housed in a large open cage (LOC: floor area 7705 cm² and height 75 cm). The LOC provided an environment with more physical, social, auditory, visual, olfactory, and tactile stimuli compared to IVCs. The aims were to compare the development of obesity comorbidities and to assess the well-being of rats housed under different conditions. The rats housed in IVCs and the LOC had similar adiposity, blood pressure, hepatic triacylglycerol content, and similar serum concentrations of cholesterol, triacylglycerol, and alanine transaminase. IVC-housed rats showed some signs of distress, such as less interest in nest-building and signs of apathy compared to LOC-housed rats. To conclude, LOC housing did not affect the typical phenotype of obese Zucker fa/fa rats but did improve the welfare of these rats.